The invention relates to aircraft auxiliary and/or emergency power systems.
The normal power requirements of an aircraft in flight are satisfied by the main engine(s) and the power system(s) connected thereto. There is a need, however, to supply power to various components of the aircraft prior to the starting, and after the stopping, of the main engine; to effectuate main engine start-up; and to handle peak power demands of the aircraft during flight operations. Moreover, an aircraft in flight may require emergency power within a few seconds after a main engine failure or after a failure of a power system driven thereby. The prior art teaches the use of a back-up power system--a so-called Emergency Power Unit (EPU)--comprising a gas generator and hot gas turbine to generate such emergency power. However, the monopropellant or bipropellant fuel typically used by such an EPU frequently carries with it the hazard of sudden, uncontrolled detonation. There are also substantial weight and cost penalties in carrying sufficient quantities of such a dedicated fuel.
Alternatively, the prior art teaches the use of a Brayton cycle gas turbine engine using JP-type fuels--a so-called Auxiliary Power Unit (APU)--to satisfy the power demands of the aircraft in an emergency. The specific fuel consumption of a JP-fueled APU is typically one order of magnitude less than that of a prior art EPU. Moreover, such an APU utilizes the same fuel as the main engine(s) and, therefore, does not require any such dedicated fuel. However, the typical APU cannot be started quickly enough to satisfy the need for emergency power. Indeed, an APU using JP-type fuel simply cannot be started or operated at the highest altitudes where modern aircraft fly due to the inability of the compressor to provide sufficient air pressure to permit ignition, or sustain combustion, of the fuel. Moreover, the maximum power output of an APU using JP-type fuel is markedly reduced as the ambient pressure and temperature external to the aircraft decrease with increasing altitude.